Self-assembling Properties of an N-Heterocyclic Carbene-based Metallosurfactant: Pd-Coordination Induced Formation of Reactive Interfaces in Water

Metallosurfactant aggregates: Structures, properties, and potentials for multifarious applications

Metallosurfactants offer important scientific and technological advances due to their novel interfacial properties. As a special class of structures formed by the integration of metal ions into amphiphilic surfactant molecules, these metal-based amphiphilic molecules possess both organometallic and surface chemistries. This review critically examines the structural transitions of metallosurfactants from micelle to vesicle upon metal coordination. The properties of a metallosurfactant can be changed by tuning the coordination between the metal ions and surfactants. The self-assembled behavior of surfactants can be controlled by selecting transition-metal ions that enhance their catalytic efficiency in environmental applications by applying a hydrogen evolution reaction or oxygen evolution reaction. We present the different scattering techniques available to examine the properties of metallosurfactants (e.g., size, shape, structure, and aggregation behavior). The utility of metallosurfactants in catalysis, the synthesis of nanoparticles, and biomedical applications (involving diagnostics and therapeutics) is also explored.

Micellar catalysis: Polymer bound palladium catalyst for carbon-carbon coupling reactions in water

Metallosurfactants, defined here as hydrophobic metal-containing groups embedded in hydrophilic units when dispersed in water, emanate in the formation of metallomicelles. This approach continues to attract great interest for its ability to serve as micellar catalysts for various metal-mediated chemical transformations in water. Indeed, relevant to green chemistry, micellar catalysis plays a preeminent function as a replacement for organic solvents in a variety of chemical reactions. There are several methods for the interaction of metal complexes (catalysts or catalyst precursors) and surfactants for producing micellar aggregates. A very effective manner for achieving this involves the direct bonding of the metal center to the amphiphilic polymeric materials. Herein, we describe the synthesis of a metallosurfactant containing a palladium complex covalently incorporated into a CO2-based triblock polycarbonate derived using a dicarboxylic acid chain-transfer agent. This amphiphilic polycarbonate was shown to self-assemble in water to provide uniform and spherical micelles, where the catalytic metal center is located in the hydrophobic portion of the micelle. The resulting metallosurfactant was demonstrated to effectively catalyze carbon-carbon coupling reactions at very low catalyst loadings.

Novel PEPPSI-Type NHC Pd(II) Metallosurfactants on the Base of 1H-Imidazole-4,5-dicarboxylic Acid: Synthesis and Catalysis in Water-Organic Media

Carrying out organic reactions in water has attracted much attention. Catalytic reactions in water with metallosurfactants, which have both a metallocenter and the surface activity necessary for solubilizing hydrophobic reagents, are of great demand. Herein we proposed new approach to the synthesis of NHC PEPPSI metallosurfactants based on the sequential functionalization of imidazole 4,5-dicarboxylic acid with hydrophilic oligoethylene glycol and lipophilic alkyl fragments. Complexes of different lipophilicity were obtained, and their catalytic activity was studied in model reduction and Suzuki-Miyaura reactions. A comparison was made with the commercial PEPPSI-type catalytic systems designed by Organ. It was found that the reduction reaction in an aqueous solution of the metallosurfactant with the tetradecyl lipophilic fragment was three times more active than the commercially available PEPPSI complexes, which was associated with the formation of stable monodisperse aggregates detected by DLS and TEM.

Metallosurfactants Consisting of Amphiphilic Ligands and Transition Metals: Structure, Bonding, Reactivity, and Self-assembling Property

Metallosurfactants are emerging as a relatively new class of surfactants whose ligand moieties bind to various transition metals. Because transition metal centers are incorporated into the surfactant frameworks, they can form various self-assembled structures with metallic interfaces such as micelles, vesicles, and lyotropic liquid crystals. To reduce the lability of transition metal complexes under aqueous conditions, various amphiphilic ligands have been developed as surfactant frameworks. This review discusses some aspects of the design and chemical structures of amphiphilic ligands, as well as focus on various functions and types of chemical bonds present in metallosurfactants.

Au(i)-, Ag(i)-, and Pd(ii)-coordination-driven diverse self-assembly of an N-heterocyclic carbene-based amphiphile

Au(i)-, Ag(i)-, and Pd(ii)-coordination-driven diverse self-assembly of an N-heterocyclic carbene (NHC)-based amphiphile was demonstrated herein. The transition metals had significant effects over the whole system, setting the self-assembly direction of the NHC-based amphiphile. More specifically, Au(i)- and Ag(i)-coordination to the NHC-based amphiphile promoted the formation of spherical and hexagonal structures, while Pd(ii)-coordination promoted the formation of cylindrical and lamellar structures.

Au(i)-, Ag(i)-, and Pd(ii)-coordination-driven diverse self-assembly of an N-heterocyclic carbene (NHC)-based amphiphile was demonstrated.

Synthesis of an N-Heterocyclic Carbene-based Au(I) Coordinate Surfactant: Application for Alkyne Hydration Based on Au Nanoparticle Formation

In this study, an N-heterocyclic carbene (NHC)-based metal coordinate surfactant (MCS), NHC-Au-MCS, in which the NHC framework afforded the bonding of the Au(I) at the linkage of the hydrophilic and hydrophobic moieties, was synthesized. The structure of NHC-Au-MCS was confirmed by ¹H and ¹³C NMR spectroscopic measurements together with elemental analysis. Matrix-assisted laser desorption/ionization (MALDI), laser desorption/ionization (LDI), and electrospray ionization mass spectrometry (ESI-MS) indicated the distinct reactivity of NHC-Au-MCS, such as the exchange of Br to Cl and the formation of a cationic Au complex, where the two NHC ligands were coordinated to an Au(I) center upon laser activation. The surface tension and dynamic light scattering (DLS) measurements revealed that the coordination of Au(I) to NHC reduced the critical micelle concentration (CMC) of NHC-Au-MCS (1.3×10^-5 M), which resulted in the formation of micelles at concentrations higher than the CMC in water. We also confirmed that the surface-active Au(I) complex of NHC-Au-MCS catalyzed the hydration of 1-dodecyne to 2-dodecanone in water in the absence of an organic solvent. On the basis of the detailed mechanistic investigations regarding the reactivity of NHC-Au-MCS, we revealed that NHC-Au-MCS partially translated into Au nanoparticles (AuNPs), which facilitated alkyne hydration. These mechanistic studies were supported by UV-vis measurements, transmission electron microscopy (TEM), and LDI-MS.